The invention relates to a process for the biological treatment of liquid wastes employing fluidized beds. In particular, it is directed to a process for removing organic carbon from waste water.
This application is related to application Ser. No. 264,346, filed June 19, 1972, now U.S. Pat. No. 3,846,289.
Sewage treatment plants are typically designed to remove solids and oxygen-demanding organic material. Traditionally, the activated sludge or trickling filter processes were employed to accomplish the required treatment incorporating the use of primary and final settling tanks for solids removal. Activated sludge treatment involves the use of microbial organisms to consume the organic wastes. The organisms are cultured as suspended solids in biologically active aeration tanks. Biological oxidation is accomplished primarily in the aeration tank where from 2,000 to 4,000 mg/l of highly active biological solids are typically maintained under aerobic conditions. Air is most often used to provide sufficient oxygen for the oxidative process as well as to keep the waste liquor well mixed with the suspended solids to prevent settling in the aeration tank. In conventional activated sludge treatment systems, about eight hours of aeration time is required to provide satisfactory removal of the organic contaminants. Following the aeration step the waste liquor with the suspended solids are conveyed into the final settling tank where the supernatant liquor is separated from the suspended solids. The liquor is normally discharged as the final effluent, while the suspended solids are partially wasted and recycled to the aeration tank where they again consume organic waste.
Traditional trickling filter processes employ four to ten foot beds of 2 to 4 inch stone, upon which biological slimes grow. Liquid waste is intermittently applied over the stones, typically by a rotary distributor which revolves around the circular trickling filter bed of stones. As waste water trickles over the biological slimes covering the stones, the organic wastes are oxidized. Air which fills the voids of the stones provides the oxygen necessary for the biological oxidation.
Both the traditional activated sludge and trickling filter processes are costly and require much space and extensive building requirements. Construction costs of the two processes are fairly comparable in small sized plants but activated sludge is often more economical for larger installations. In general, these processes provide a substantial reduction of the 5-day biochemical oxygen demand (BOD).sub.5 and suspended solids present in, for example, municipal waste water. These processes, in effect, convert the organic contaminant, typically measured as BOD.sub.5, to innocuous carbon dioxide and water; but also produce biological mass which must be given further treatment. Such processes are highly expensive and require much land, personnel and lead time to develop- to meet today's critical needs for compact water purification systems.
New activated sludge processes in the developmental stages use pure oxygen and suspended solids of 4,000 to 5,000 mg/l in the aeration tanks, but still require from 2 to 4 hours aeration time, as well as bulky tanks and piping systems.
In the past, experimenters have employed up-flow expanded beds operating under anaerobic conditions and containing activated carbon for the adsorption of minor amounts of organic carbon (BOD) that remained after conventional biological treatment or physical/chemical treatment. Such adsorption processes employing expanded beds had not proved satisfactory or feasible for large scale waste water purification systems. Frequent back-washing of the system is needed as the pores in the activated carbon tend to become rapidly filled with contaminants.
While the art has recognized the desirability of employing biological organisms to remove organic carbon, it has not succeeded in providing an inexpensive and highly efficient process for rapidly treating large quantities of waste water. Accordingly, there exists a critical need for a process free of the defects and deficiencies of the prior art to purify waste water.
As employed in the application the term "waste water" or "liquid waste" includes organic or inorganic liquids or mixtures thereof containing biologically decomposable contaminants. Preferably, the waste water to be processed contains the equivalent of at least about 30 milligrams per liter of biochemical oxygen demand (BOD); particularly in the organic carbon form of BOD. Almost all municipal and industrial waste water which contain BOD fall within the above definition.